Systems for distributing information to subscriber substations have been known for some time, but their limitations have inhibited the growth of cable communications. In the past, such systems have delivered television signals via a main cable that transmits a main video signal carrying the television information. In some such cables, the cable conductor also has carried a 60 cycle AC main power signal in order to provide power for amplifiers used to amplify weak video signals on the cable.
Recent developments in cable technology have placed increased demands on the ability of the cable to provide not only video signals, but also power signals to subscriber substations. As far as the applicants are aware, no one has successfully developed a tap capable of providing both a video signal and a power signal to a plurality of subscriber substations. Obtaining AC power signals from a main cable transmitting both video and power signals is complicated by the increased bandwidth required for improved cable video operation. At the present time, such a bandwidth typically extend from about 5 megahertz to over 1 gigahertz.
The applicants have discovered techniques for supplying AC power signals to a multiplicity of subscriber substations with only a minimal loss to the video signal. For example, in a typical tap for supplying subscriber substations, the applicants have been able to supply power from the cable power signal with an increase in insertion loss of only about 0.2 decibels of radio frequency energy from the video signal when compared to a tap that does not supply power to the substation.
In addition to supplying a power signal to multiple subscriber substations with minimal loss to video energy, there also is a need for a tap capable of supplying both video and power signals to subscriber substations, irrespective of the direction in which the video signal is transmitted on the main cable, without rewiring existing taps or drop lines to the subscriber substations.
There also is a need for limiting current supplied by power signals to subscriber substations and limiting the amount of radio frequency electromagnetic energy that is emitted by a tap serving subscriber substations.
The cable television industry has long sought a means of quickly connecting a cable conductor to a tap without creating variable capacitance and without requiring undue labor by an installer. In addition, the coupling technique must match as nearly as possible the characteristic impedance of the cable conductor.
Each of the foregoing deficiencies in the prior cable television taps has been overcome by using the unique techniques described and claimed in the specification.